1. Field of the Invention
The present invention relates to an automobile antenna system, and more particularly, to an automobile antenna system which performs diversity reception of broadcast waves by detecting using, a plurality of high-frequency pickups, the surface currents induced on the vehicle body by the broadcast waves.
2. Description of the Prior Art
Antenna systems are indispensable to automobiles which must positively receive various broadcast waves such as those for radio, television and telephone at the receivers located within the vehicle. Such antenna systems are also very important for citizen band transceivers. Therefore, such antenna systems play an important role in the communication equipment which will be installed as a factory-installed item in the future.
In particular, television (TV) sets are these days often installed in automobiles, and it has become increasingly important for such automobiles to have an automobile antenna system capable of positively receiving TV broadcast waves.
However, high-frequency waves in the VHFL0 band (90 to 108 MHz), VHFHi band (170 to 220 MHz), or UHF band (440 to 770 MHz), such as TV broadcast waves, have a strong tendency to suffer from rectilinear propagation, so that direct waves and other waves interfere with each other as a result of bouncing off buildings, hill sides or other obstacles and this phenomenon produces distortion. Furthermore, in the case of a vehicle which is parked, the disturbance created by the waves generated by passing vehicles can cause multipath noise such as momentary interruption of sound during the reception of the TV broadcast waves. An automobile antenna system having only a single antenna is unable to reduce such multipath noise during the reception of TV broadcast waves.
Furthermore, it is difficult to receive TV broadcast waves stably, even if only the sound of the TV broadcast is received, by a conventional pole antenna during the travel of a car, because, due to the directivity of the antenna, the receiving state of the antenna changes in correspondence with the change in direction from which the broadcast waves come, during the travelling of the automobile.
To solve this problem, a diversity-reception type automobile TV antenna system has been known, in which at least two antennas are disposed on the vehicle body at predetermined intervals so that receiving operation is taken over by the antenna which enjoys superior reception by automatic changeover, whereby the directivity of the antenna is improved and the amount of multipath noise is lowered.
However, a conventional antenna is generally a pole antenna which projects outwardly from the vehicle body, and although it is superior in performance in its own way, it always remains a nuisance from the viewpoint of vehicle body design.
Especially, when diversity reception is preformed by an automobile antenna system, a plurality of antennas are newly required. As a result, the aesthetic appearance of the automobile is disadvantageously damaged, and the receiving performance is greatly deteriorated by electrical interference between the antennas
Another type of improved automobile antenna system for radio broadcast reception has been proposed which performs diversity reception system by a pair of rear quarter glass antennas consisting of antenna elements which are pasted to the glass of the rear quarter window (Japanese Patent Laid-Open No. 61509/1983).
In this automobile antenna system, the receiving operation is taken over by the antenna which enjoys superior reception, two antennas being disposed on the rear quarter glass to the right and the left.
In such a diversity reception system composed of a pair of rear quarter glass antennas, one can compensate for the other if the receiving state of the latter deteriorates with respect to the broadcast waves travelling in the lateral direction relative to the advancing direction of the vehicle. On the other hand, with respect to broadcast waves travelling parallel to the advancing direction of the vehicle, the two antennas cannot together compensate adequately for a deteriorated receiving state.